1. Field of the Invention
The present invention relates to an optical pick-up for reading information recorded on an optical disk such as a DVD (Digital Versatile Disk) and more particularly to an optical pick-up for reading an optical disk having a plurality of recording layers.
2. Description of the Related Art
Since the DVD as one kind of the optical disk has a large storage capacity, the DVD is employed for various kinds of uses as a media for recording images or audio. The DVD has recording layers on which information is recorded and the information recorded on the recording layers is optically read by the optical pick-up. That is, the information recording surfaces of the recording layers are irradiated with an optical beam emitted from a light emitting element (semiconductor laser) provided in the optical pick-up and the reflected light thereof is received by the light receiving element of the optical pick-up. Then, a signal process is performed to reproduce the recorded information of the DVD. Further, recently, a DVD having two recording layers has been mainly developed to more increase the storage capacity.
In order to read the information recorded on the optical disk by the optical pick-up, the spot of the optical beam needs to be accurately converged on the information recording surface of the optical disk. Accordingly, the optical pick-up is provided with a movable objective lens. When the optical disk having the two recording layers is read, the objective lens is moved to meet the recording layer to be read to perform a focusing operation so that light emitted from the lens is focused on the recording layer to be read.
FIGS. 4A and 4B are diagrams showing an optical system of a usual optical pick-up having such an objective lens. In FIGS. 4A and 4B, 1 designates an optical disk composed of a DVD and having two recording layers including a first recording layer 1a and a second recording layer 1b on which information is recorded. 50 designates a light source that applies a laser beam to the optical disk 1. 51 designates a collimate lens for converting the laser beam 53 into parallel lights 53c. 52 designates a movable objective lens for converging the parallel lights 53c from the collimate lens 51 on the recording layers 1a and 1b of the optical disk 1. An infinite optical system of the optical pick-up is formed with these parts.
In FIG. 4A, converged lights 53a from the objective lens 52 form a focal point F1 on the recording layer 1a to read the information recorded on the recording layer 1a by the optical pick-up. When the recording layer is changed to read the information recorded on the recording layer 1b, the objective lens 52 is moved upward (in a direction of C) as shown in FIG. 4B. At this time, a moving distance y is a distance corresponding to a space between the recording layer 1a and the recording layer 1b. Thus, converged lights 53b from the objective lens 52 form a focal point F2 on the recording layer 1b to read the information recorded on the recording layer 1b by the optical pick-up.
The optical pick-up for reading the optical disk having a plurality of recording layers by the above-described infinite optical system is disclosed in, for instance, a below-described JP-A-2002-373444. Further, JP-A-2002-373442 discloses an optical pick-up composed of a semi-finite optical system having an intermediate lens for converging an optical beam between a light source and a beam splitter or an optical pick-up that uses an optical system including a first optical element between a light source and a beam splitter and a second optical element between the beam splitter and an objective lens.
However, in the case of the optical pick-up using the infinite optical system as shown in FIGS. 4A and 4B, the collimate lens 52 is necessary, so that the number of parts is disadvantageously increased to raise a cost. On the other hand, in the case of the optical pick-up disclosed in the JP-A-2002-373442, the collimate lens is not necessary, however, the intermediate lens or the optical elements are necessary in place thereof. Accordingly, the number of parts is still large and a cost cannot be avoided from increasing.
As compared therewith, in an optical pick-up for reading a CD (Compact Disk), a finite optical system that an optical beam emitted from a light source is allowed to be directly incident on an objective lens and converged on the disk is employed. Thus, a method may be considered that such a finite optical system is used for an optical pick-up for a DVD to save a collimate lens. Specifically, in the finite optical system, since only one focus of the objective lens is fixed, a problem does not arise in the CD that has basically one recording layer. However, in the DVD having a plurality of recording layers, a reproducing accuracy is greatly different depending on which of the layers a focal point is set to. The above-described situation arises because of a reason why the quantity of reflected light is different respectively in the recording layers due to the attenuation of light or the like. Thus, even when the focal point is slightly diffused, information can be read in the recording layer having a large quantity of reflected light, however, when a focal point is diffused, information is extremely hardly read with accuracy in a recording layer having a small quantity of reflected light. Accordingly, when a technique of the optical pick-up for the CD is simply diverted to that of the optical pick-up for the DVD, technical problems to be solved still remain.